The present invention relates to a process for manufacturing a valve seat which may suitably be used for an internal combustion engine, made of a sintered Fe alloy and to a valve seat made of a sintered Fe alloy, and more specifically to the process for manufacturing the valve seat and the valve seat wherein the valve seat comprises a contacting layer abutting against a valve and a main layer supporting the contacting layer.
A valve seat has a conical surface portion to which a valve is abutted and is a component part of an internal combustion engine. The valve seat is formed by processing of a cylinder material or a cylinder head casting material and by fit of a ring formed beforehand. In either case, the valve seat is required to improve a thermal conductivity, lubrication property and a strength at a high temperature.
Hitherto, there has generally been used the valve seat to which an infiltration treatment of Cu is performed into a sintered Fe alloy in order to solve the problem described above. As a process for manufacturing the conventional valve seat, for example, a valve seat without including both Cu and Pb is sintered by a normal sintering method in the first place, and the valve seat with both Cu and Pb is so obtained that holes in the sintered Fe alloy is infiltrated by either Cu or Pb in the final place. Thus, the thermal conductivity, the lubrication property and the strength at a high temperature thereof have been improved.
However, there is a problem that the manufacturing cost of the valve seat made of the sintered Fe alloy with either infiltrated Cu or infiltrated Pb becomes high because the infiltration treatment is needed in addition to the sintered treatment.
In view of the above-mentioned circumstances, there is known from Japanese Patent Publication No. 2-35125 a process for manufacturing a valve seat with two green compact body layers made of a sintered Fe alloy to provide the valve seat which has the same property as the conventional valve seat and is cheap to manufacture. The valve seat described above is obtained by the step of formation of a green compact body comprising two green compact body layers in which one green compact body layer is a main layer with either Cu or Pb and the other green compact body layer is a contacting layer without both Cu and Pb, and by the next step of sintering of the green compact body. According to the process described above, the property of the contacting layer which is contacted to the valve, can be improved by infiltration of either Cu or Pb from the main layer to the contacting layer at the same time of the sintering process without the independent infiltration treatment of both Cu and Pb.
The process described above has an excellent property because the contacting layer after the sintering process becomes the infiltrated layer of either Cu or Pb without the porosity which exists in the layer before the sintering process. The infiltration by capillarity of either Cu or Pb is carried out from the main layer to holes in the contacting layer at the same time of the sintering process. However, the process has a problem that holes are generated in the main layer by the infiltration of either Cu or Pb, so that a strength of the main layer decreases, and a whole strength of the valve seat also decreases.
And, Pb may give a bad influence to the environment.